Most current automotive passenger vehicles include at least one inflatable airbag as part of the occupant protection system. Some proposed occupant protection systems combine an upper body airbag and a knee airbag. An upper body airbag is mounted to the instrument panel (IP) forward of a seated occupant at a relatively high position and deploys or inflates rearwardly to contact the occupant's chest or upper thorax region. A knee airbag is mounted to a lower position on the instrument panel and inflates or deploys to contact the occupant in the area of the knees and/or shins Recognized benefits of knee airbags include providing cushioning of any impact between the knees/lower legs and the lower portion of the instrument panel, as well as prevention of submarining, in which a seat occupant's hips move forwardly beneath a lap and/or chest restraint belt.
Examples of such knee airbag systems include U.S. patent applications 2004/0124617 Al and U.S. 2007/0267852 Al and U.S. Pat. No. 6,712,384. The '384 patent teaches that an upper body airbag and a knee airbag may be used in combination and that completing the expansion of the knee airbag earlier than the expansion of the body airbag may have benefits.